Sheet-type ohmic heating elements have the advantage of being flexible so that they can be mounted on a surface of any virtually any shape.
A PTC material is a material the specific electric resistance of which rises with increasing temperature. The temperature coefficient indicates the increase in resistance per unit of temperature increase. A heating element having such a PTC characteristic self-regulates the heat that it emits. As an electrical current is caused to flow across the heating element, the temperature of the heating element rises. Due to the increasing resistance, the electrical current is reduced until equilibrium is reached.
In general, power controlled heating devices, such as e.g. seat-heater (SH) devices, require electronic control units in order to establish a set of well-defined heating power levels. In such cases heating control is either done directly via thermostat elements in the supply circuit of the actual heating element or by using a pulsing electronics which regulates the mean heater current by varying the relative ON/OFF time interval of the power supply.